When it gets to an outside collection agency, that agency will send you a notice that they are handling the collections. If you haven't received a notice tell them you would like one mailed to you so you may verify it with to whom the debt is owed.
Here is how most collection calls go:
1. Confirm person is debtor
2. mini-Miranda (yes they have to say they are a debt collector and anything said is for that purpose)
3. annoy the piss out of you about payment and reconfirmation of contact information
Play hardball, if you offer monthly installments, make sure it is lower than what you want to pay, they will always try to get you to pay a higher amount. If you are gonna pay $20/month, offer $10 and negotiate to $20.
Also don't pay the full amount, make some excuse like you are not paying those ridiculous/unwarranted fees that were tacked on. Most collector's are capable of lowering the total (by how much varies from account to account) but the collector gets payed on percentage. Once again start low saying "the bill was $100, the rest are fees which they can shove" and negotiate as low as possible.
They like to say "We may have to send this to legal" means squat, in fact ask them what that means. Ask them if that was a threat to sue you. "This may effect your credit score" means squat again. Ask "Are you reporting this to a credit bureau?" and "Which credit bureau are you reporting this to?" Another empty intimidation line is "I think you should consult a lawyer." If you have any debt that is bad to the point of an outside collection agent is calling, you really may need to consult a lawyer. For the sake of knowing where the collector broke laws. A fun one to pull there is "I was going to be contacting (insert name of state attorney general here, good to have memorized) tomorrow about a certain collection call that I feel MAY be harassing to see if he had any advise."
The use of the word "may" is important, they will never say "we will do this" and neither should you. They always say "We MAY do this."
They play hardball with you, play it with them.
A tip to cause them headaches.
Have your roommate answer the phone and have them ask if they are a debt collector or collecting on a debt. If they say yes, they broke the FDCPA. If they say no it is gray area being that misrepresentation can also violate the FDCPA.
Also, read up on the Fair Debt Collection Practices Act (FDCPA). It is there to protect you.